Inductive speed sensor having switching hysteresis for increasing the magnitude of a phase shift

Abstract

A speed detector, for use with a tone wheel having metal teeth, employs two coils positioned in offset relation near the tone wheel for interacting with the tone wheel teeth and thus changing their respective inductances. One coil is in a phase sense oscillator circuit and the other is in a tuned sensor network. The oscillator is coupled to, and drives, the tuned network. The change in coil inductances effects a shifting of the phase of the output signal from the oscillator relative to the output signal from the tuned network. The phase sequence of those two output signals also reverses itself cyclically as the tone wheel rotates, and a phase reversal detector detects that reversal and uses the event to provide timing reference pulses for a final determination of speed. The speed detector has a hysteresis circuit connected between the phase reversal detector and the oscillator or the tuned network for automatically increasing the apparent phase shift between the oscillator and tuned circuit output signals to minimize invalid timing signals as a result of limited backward motion of the tone wheel. The hysteresis circuit includes a resistor and preferably a series diode.

I claim: 
     
       1. A speed detector apparatus for use in combination with a tone wheel having multiple metal teeth, the detector apparatus and tone wheel being adapted for relative displacement and further being susceptible to relative backward displacement, comprising: an oscillator including a first inductive sensing coil adapted to be positioned adjacent to passing teeth on said tone wheel to thereby change inductance, the oscillator having a first output signal having a frequency determined at least partly by the inductance of said first coil, the oscillator further including means for providing said first output signal as a digital pulse waveform, the duty cycle of said digital waveform being at least partly a function of the DC voltage of the oscillator;   a tuned circuit interconnected with said oscillator and including a second inductive sensing coil, said tuned circuit being driven by said oscillator to provide a second output signal and said second inductive sensing coil being adaptive to be positioned adjacent to passing teeth on said tone wheel to thereby change inductance, the tuned circuit including means for providing said output signal as a digital pulse waveform, the duty cycle of said digital waveform being at least partly a function of the DC voltage of the tuned circuit;   said first and said second inductive sensing coils being offset from one another relative to the passing teeth on the toned wheel, such that their respective said changes of inductance are relatively out-of-phase;   the phase of said second output signal relative to said first output signal shifting and reversing cyclically as a function of the relative positioning of said teeth to the respective said first and second sensing coils during said relative displacement of the tone wheel;   means coupled to said oscillator and said tuned circuit for detecting a said reverse of said phase of one of said first and said second output signals relative to the other and thereby providing timing reference signals recurring at intervals representative of the relative speed of said tone wheel; and   hysteresis circuit means connected between said phase-reversal detecting means and at least one of said oscillator and said tuned circuit for automatically adjusting said DC voltage of the respective said oscillator or said tuned circuit upon said detection of a phase reversal to thereby increase the magnitude of the phase shift.   
     
     
       2. The speed detector apparatus of claim 1, wherein said hysteresis circuit means adjusts the level of DC voltage in both said oscillator and said tuned circuit to increase the magnitude of the said phase shift. 
     
     
       3. The speed detector apparatus of claim 2, wherein said phase-reversal detecting means is a bistable logic element having complementary outputs and said hysteresis circuit means is connected to one of said outputs of said phase-reversal detecting means and includes resistance means for limiting current flow. 
     
     
       4. The speed detector apparatus of claim 3, wherein said hysteresis circuit means further includes unidirectional conducting means in series with said resistance means. 
     
     
       5. The speed detector apparatus of claim 4, wherein said unidirectional conducting means is a diode, said diode being connected for conduction when said bistable element output is substantially at ground potential to thereby lower the DC voltage in the respective said oscillator or tuned circuit to which said hysteresis circuit means is connected. 
     
     
       6. The speed detector apparatus of claim 5, wherein said resistance means limits the amount by which said DC voltage is lowered.